Coating compositions including polyurethane (PU) polymers generally exhibit excellent resistance to abrasion, chemicals, and solvents. PU coatings may be used in various applications such as, for example, coatings for wood, concrete, metal, semi-rigid and flexible plastics, rubber, and leather, as well as in glass fiber sizing, printing inks, and adhesives.
PU coating compositions that exhibit suitable mechanical properties for such applications may be prepared using oil-modified PU polymers. Conventional oil-modified PU polymers are typically prepared in organic solvents and applied in clear or pigmented coatings. After application of a PU coating composition to a substrate, the organic solvent is evaporated to form a film that is then typically cured by air oxidation of ethylenic groups included in the PU polymers of the film.
Governments have increasingly established regulations restricting the release of volatile organic compounds (VOCs) into the atmosphere, which impact the use of PU coating compositions containing oil-modified PU polymers. To reduce the amount of released VOCs, manufacturers have been reducing the amount of organic solvent in PU coating compositions through use of water-dispersible PU polymers. Conventional water-dispersible PU polymers may be produced, for example, by reacting polyols and dihydroxy carboxylic acid compounds with an excess of diisocyanate to provide a carboxy-functional prepolymer having isocyanate (NCO) terminal groups. The acid groups may be neutralized to provide a neutralized prepolymer that is dispersible in water, which may be further modified to elicit various properties.
PU films formed using conventional water-dispersible PU polymers typically vary from films that are hard and relatively inflexible to films that are soft and highly flexible. It is often difficult to prepare a PU film that exhibits a proper blend of performance characteristics (e.g., both good hardness and flexibility) from conventional water-dispersible PU polymers. To achieve both good hardness and flexibility using conventional water-dispersible PU polymers, manufacturers typically formulate the PU polymers using increased amounts of isocyanate, which may result in increased material costs that are prohibitive for various applications.
As such, there is a continuing need for new aqueous-based low VOC or substantially VOC-free PU coating systems. There is also a need for such aqueous coating compositions that cure by crosslinking upon the evaporation of the water.